Water resource implications of O-18 and H-2 distributions in a basalt aquifer system

Ongoing decline of water levels in the confined basalt aquifers of the Pull man-Moscow Basin of Washington and Idaho has prompted study of the timing, amount and distribution of recharge to the system. Previous radiocarbon age s indicate residence times on the order of 10(3) years and greater and sugg est a low rate of recharge to the lower basalt aquifer since the end of Ple istocene time. By contrast, more recent hydrodynamic flow modeling studies invoke a larger Holocene recharge rate through the unconfined loess unit to the upper and lower basalt aquifers, which implies relatively short reside nce times (10(2) years). Stable isotopes were used to independently assess contrasting recharge models by comparing O-18/O-16 and D/H ratios of late-H olocene shallow ground water and deep ground water. Linear regression of lo cal precipitation ratios yields deltaD = 6.9 delta O-18 - 18.4. There is no evidence of fractionation of ground water ratios by recharge processes or water-rock interactions. Deep basalt ground water delta O-18 values are dep leted by 0.4 to 4.9 per mil relative to shallow, recently recharged ground waters and have delta O-18 values statistically distinct from waters sample d from other stratigraphic units. These findings suggest that the deep wate rs in the basin were not precipitated under current climate conditions and that aquifer recharge rates to the deep basalt aquifer are substantially lo wer than have been recently estimated. This in turn suggests that a sustain able ground water exploitation scheme must reduce reliance on the deep grou nd water resource.